Borehole fluid samples collected from Site 504B, during ODP Leg 137, provide the best opportunity to date to study water-rock interaction within the ocean crust. Chemical composition of the borehole fluids reveal the results of seawater-basalt interaction at elevated temperatures. The borehole fluids can be described as a two component mixture, consisting of a reacted fluid component and a seawater which has convectively mixed into the borehole from the seafloor. Progressive trends in fluid chemistry with time suggest that the fluid-rock reactions have changed as the borehole has been drilled deeper into the hotter formation. Based on data from this study and comparisons to fluid studies from past visits to Site 504B, indications are that these reactions are occurring within the borehole environment. ¿ American Geophysical Union 1992